1. Field of the Invention
The invention relates generally to a multiple node ring communications system, and more particularly to an optical fiber multiple node ring communications system including a dual optical fiber between nodes and a communications method for an optical fiber ring communications system.
2. Description of the Prior Art
Fiber optic communications systems have the potential for providing large system bandwidth and high information carrying capacity. Increased capacity would be very desirable for many types of communications systems. A significant example is data communications where a need exists for rapidly and reliably transmitting large volumes of digital information.
Many types of laser-based devices and systems, having a wide range of applications, such as in medical technology, in communications and computing technology, are becoming increasingly well known and commercially available.
The lasers used in many of these devices and systems are often capable of producing powerful outputs that are potentially harmful to both people and equipment. As a result, many types of safety devices for use in conjunction with laser-based equipment, and standards designed to ensure that laser-based equipment may be safely operated, have been developed and continue to evolve.
For example, U.S. Pat. No. 5,136,410 to Heiling et al. describes an optical fiber link control safety system that operates, in a self-contained fashion, as part of an optical fiber link card or optical link module (OLM). The disclosed optical fiber control (OFC) circuitry reduces or shuts off a laser's radiant energy output when a link failure is detected and periodically checks to determine if the failure is corrected and restores full continuous power to the laser when determined that the line is operationally safe.
ANSI standards require an OFC circuit for all shortwave laser communications devices. Typically shortwave lasers are used in fiber channel devices. Shortwave lasers can be used in asynchronous transfer mode (ATM) protocol communications cards, so the OFC circuit can also be used in ATM protocol communications cards. There is also the potential for shortwave lasers to be used in fiber-distributed data interface (FDDI) protocol communications cards, thus again requiring use of the OFC circuit for safety purposes. Also, the OFC circuit can be adapted for use in longwave laser communications cards to provide a less expensive alternative than the dual shutter method typically used for safety in longwave laser applications.
In conventional single optical fiber ring communications systems, typically only five or six nodes can be put in the ring and the total ring circumference must be less that two kilometers. This five or six computer, two kilometer limitation is due to the characteristics of the OFC circuit. The known optical fiber ring communications systems include multiple nodes with each node having a transmitter and a receiver connected to a respective one of two optical fibers connected to the node and a single optical fiber between adjacent nodes. A single closed path or unidirectional communications ring or loop is provided. Upon power-up and periodically during operation, the optical fiber control (OFC) circuitry used to detect when a link failure has occurred and to periodically check to determine if the failure has been corrected, requires a test signal to propagate once around the ring. This test signal is referred to as a loop-is-closed signal. The OFC circuitry of one of the nodes sends a loop-is-closed signal from its transmitter and waits for the signal to be relayed through each node around the ring back to its receiver. If the loop-is-closed signal does not return to its receiver within a predetermined time period, the OFC circuitry determines that a link failure has occurred and shuts the laser off to avoid a potential safety hazard. As the distance and the number of OFC cards increases, the loop-is-closed signal takes longer and longer to go around the ring. When the number of nodes increases so that the loop-is-closed signal is unable to complete the loop within the predetermined time period, the OFC will not allow any of the cards to power up their lasers for normal communications. Another significant disadvantage of conventional single optical fiber ring communications systems is that the entire loop goes down when one fiber becomes inoperative.
A need exists for a communications method and optical fiber apparatus that avoids the inherent limitations resulting from an OFC circuit (five or six computers and two kilometer ring limitations) of the prior art systems and that provides improved performance and is simple to implement.